Nearly two decades of epidemiological and clinical studies have documented the staggering rates of comorbidity between trauma exposure and substance use, as well as the high rates of PTSD comorbidity. However, treatments which effectively address these complex symptoms are still at an early stage of development. This study will enhance the knowledge and understanding of concurrently treating individuals with substance use disorders and PTSD in hopes of facilitating treatment retention and increasing the likelihood for more positive treatment outcomes. Following NIDA's stage model for behavior therapies development, this study will take the next step building upon promising findings from Stage IA work with an exposure-based therapy modified to use safely with substance dependent individuals. The first aim of this Stage1B clinical trial is to modify and test an exposure-based treatment for individuals with substance use disorder and comorbid PTSD. Using a two-armed randomized design, the proposed clinical trial will assess the relative efficacy of: 1) Concurrent Treatment of PTSD and Substance Dependence (CTPSD) which incorporates exposure techniques with CBT, and 2) Relapse Prevention Treatment (RPT). Participants will be recruited from the greater New York City area, with study procedures taking place at St. Luke's\Roosevelt Hospital Center. Treatment will consist of 12 individual sessions with repeated measures at baseline, end of treatment, and 1-, 2- and 3-months post-treatment. The primary outcome is PTSD symptom severity. Secondary outcomes are (2a) substance abuse severity, (2b) global psychiatric symptom severity, and (2c) treatment retention and compliance. The third exploratory aim of the proposed trial is to test the potential mechanisms of action in the respective treatments. Based on findings regarding the neurobiology of trauma, we hypothesize that emotion regulation and related physiological processes are predictive of treatment outcome. We will test for mediator and moderator effects using both subjective (self-report) and objective (electrodermal skin fluctuation) measures. The fourth exploratory aim of the proposed trial is to explore pathways between the comorbid PTSD and SUD diagnoses and how they may change through the course of treatment. The proposed study has the particular advantages of building directly on positive preliminary Stage IA empirical findings by (1) further testing exposure therapy in individuals with PTSD and SUDs using a randomized design with an active treatment comparison group, (2) measuring emotion regulation and physiological arousal with both subjective and objective measures, as well as measuring PTSD and SUD outcomes, and (3) utilizing a multi-disciplinary team of experts who have successfully collaborated in the past and are uniquely qualified to design and implement this type of investigation.